PURPOSE: To control the outgoing light quantity of two laser diodes by one photodetector.

CONSTITUTION: This circuit comprizes a photodetector 16 detecting the summed light quantity of the respective outgoing lights of laser diodes 12, 13, a current control circuit 19 supplying a first driving current to the laser diode 12 and a current control circuit 20 supplying a second driving current turned on/off by a first control signal, adding it to the first driving current, to the laser diode 12. The circuit comprizes also a current control circuit 22 supplying a third driving current turned on/off by a second control signal to the laser diode 13 and detector circuits 24, 25, 26, 27, 28 detecting the output signals of the photodetector 16 when the first and the second control signals show the on/off states. The current control circuits 19, 20 22 adjust the first to the third driving currents basing on either of the output signals of the detector circuits, respectively.